﻿// Copyright (c) Microsoft. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
// ProducerConsumerQueues.cs
//
//
// Specialized producer/consumer queues.
//
//
// ************<IMPORTANT NOTE>*************
//
// There are two exact copies of this file:
//  src\ndp\clr\src\bcl\system\threading\tasks\producerConsumerQueue.cs
//  src\ndp\fx\src\dataflow\system\threading\tasks\dataflow\internal\producerConsumerQueue.cs
// Keep both of them consistent by changing the other file when you change this one, also avoid:
//  1- To reference internal types in mscorlib
//  2- To reference any dataflow specific types
// This should be fixed post Dev11 when this class becomes public.
//
// ************</IMPORTANT NOTE>*************
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

using System.Collections;
#if CONCURRENT_COLLECTIONS
using System.Collections.Concurrent;
#else
using System.Threading.Tasks.Dataflow.Internal.Collections;
#endif
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.Contracts;
using System.Runtime.InteropServices;

namespace System.Threading.Tasks
{
	/// <summary>Represents a producer/consumer queue used internally by dataflow blocks.</summary>
	/// <typeparam name="T">Specifies the type of data contained in the queue.</typeparam>
	internal interface IProducerConsumerQueue<T> : IEnumerable<T>
	{
		/// <summary>Enqueues an item into the queue.</summary>
		/// <param name="item">The item to enqueue.</param>
		/// <remarks>This method is meant to be thread-safe subject to the particular nature of the implementation.</remarks>
		void Enqueue(T item);

		/// <summary>Attempts to dequeue an item from the queue.</summary>
		/// <param name="result">The dequeued item.</param>
		/// <returns>true if an item could be dequeued; otherwise, false.</returns>
		/// <remarks>This method is meant to be thread-safe subject to the particular nature of the implementation.</remarks>
		bool TryDequeue(out T result);

		/// <summary>Gets whether the collection is currently empty.</summary>
		/// <remarks>This method may or may not be thread-safe.</remarks>
		bool IsEmpty { get; }

		/// <summary>Gets the number of items in the collection.</summary>
		/// <remarks>In many implementations, this method will not be thread-safe.</remarks>
		int Count { get; }

		/// <summary>A thread-safe way to get the number of items in the collection. May synchronize access by locking the provided synchronization object.</summary>
		/// <param name="syncObj">The sync object used to lock</param>
		/// <returns>The collection count</returns>
		int GetCountSafe(object syncObj);
	}

	/// <summary>
	/// Provides a producer/consumer queue safe to be used by any number of producers and consumers concurrently.
	/// </summary>
	/// <typeparam name="T">Specifies the type of data contained in the queue.</typeparam>
	[DebuggerDisplay("Count = {Count}")]
	internal sealed class MultiProducerMultiConsumerQueue<T> : ConcurrentQueue<T>, IProducerConsumerQueue<T>
	{
		/// <summary>Enqueues an item into the queue.</summary>
		/// <param name="item">The item to enqueue.</param>
		void IProducerConsumerQueue<T>.Enqueue(T item) { base.Enqueue(item); }

		/// <summary>Attempts to dequeue an item from the queue.</summary>
		/// <param name="result">The dequeued item.</param>
		/// <returns>true if an item could be dequeued; otherwise, false.</returns>
		bool IProducerConsumerQueue<T>.TryDequeue(out T result) { return base.TryDequeue(out result); }

		/// <summary>Gets whether the collection is currently empty.</summary>
		bool IProducerConsumerQueue<T>.IsEmpty { get { return base.IsEmpty; } }

		/// <summary>Gets the number of items in the collection.</summary>
		int IProducerConsumerQueue<T>.Count { get { return base.Count; } }

		/// <summary>A thread-safe way to get the number of items in the collection. May synchronize access by locking the provided synchronization object.</summary>
		/// <remarks>ConcurrentQueue.Count is thread safe, no need to acquire the lock.</remarks>
		int IProducerConsumerQueue<T>.GetCountSafe(object syncObj) { return base.Count; }
	}

	/// <summary>
	/// Provides a producer/consumer queue safe to be used by only one producer and one consumer concurrently.
	/// </summary>
	/// <typeparam name="T">Specifies the type of data contained in the queue.</typeparam>
	[DebuggerDisplay("Count = {Count}")]
	[DebuggerTypeProxy(typeof(SingleProducerSingleConsumerQueue<>.SingleProducerSingleConsumerQueue_DebugView))]
	internal sealed class SingleProducerSingleConsumerQueue<T> : IProducerConsumerQueue<T>
	{
		// Design:
		//
		// SingleProducerSingleConsumerQueue (SPSCQueue) is a concurrent queue designed to be used 
		// by one producer thread and one consumer thread. SPSCQueue does not work correctly when used by 
		// multiple producer threads concurrently or multiple consumer threads concurrently.
		// 
		// SPSCQueue is based on segments that behave like circular buffers. Each circular buffer is represented 
		// as an array with two indexes: _first and _last. _first is the index of the array slot for the consumer 
		// to read next, and _last is the slot for the producer to write next. The circular buffer is empty when 
		// (_first == _last), and full when ((_last+1) % _array.Length == _first).
		//
		// Since _first is only ever modified by the consumer thread and _last by the producer, the two indices can 
		// be updated without interlocked operations. As long as the queue size fits inside a single circular buffer, 
		// enqueues and dequeues simply advance the corresponding indices around the circular buffer. If an enqueue finds 
		// that there is no room in the existing buffer, however, a new circular buffer is allocated that is twice as big 
		// as the old buffer. From then on, the producer will insert values into the new buffer. The consumer will first 
		// empty out the old buffer and only then follow the producer into the new (larger) buffer.
		//
		// As described above, the enqueue operation on the fast path only modifies the _first field of the current segment. 
		// However, it also needs to read _last in order to verify that there is room in the current segment. Similarly, the 
		// dequeue operation on the fast path only needs to modify _last, but also needs to read _first to verify that the 
		// queue is non-empty. This results in true cache line sharing between the producer and the consumer.
		//
		// The cache line sharing issue can be mitigating by having a possibly stale copy of _first that is owned by the producer, 
		// and a possibly stale copy of _last that is owned by the consumer. So, the consumer state is described using 
		// (_first, _lastCopy) and the producer state using (_firstCopy, _last). The consumer state is separated from 
		// the producer state by padding, which allows fast-path enqueues and dequeues from hitting shared cache lines. 
		// _lastCopy is the consumer's copy of _last. Whenever the consumer can tell that there is room in the buffer 
		// simply by observing _lastCopy, the consumer thread does not need to read _last and thus encounter a cache miss. Only 
		// when the buffer appears to be empty will the consumer refresh _lastCopy from _last. _firstCopy is used by the producer 
		// in the same way to avoid reading _first on the hot path.

		/// <summary>The initial size to use for segments (in number of elements).</summary>
		private const int INIT_SEGMENT_SIZE = 32; // must be a power of 2
		/// <summary>The maximum size to use for segments (in number of elements).</summary>
		private const int MAX_SEGMENT_SIZE = 0x1000000; // this could be made as large as Int32.MaxValue / 2

		/// <summary>The head of the linked list of segments.</summary>
		private volatile Segment _head;
		/// <summary>The tail of the linked list of segments.</summary>
		private volatile Segment _tail;

		/// <summary>Initializes the queue.</summary>
		internal SingleProducerSingleConsumerQueue()
		{
			// Validate constants in ctor rather than in an explicit cctor that would cause perf degradation
			Debug.Assert(INIT_SEGMENT_SIZE > 0, "Initial segment size must be > 0.");
			Debug.Assert((INIT_SEGMENT_SIZE & (INIT_SEGMENT_SIZE - 1)) == 0, "Initial segment size must be a power of 2");
			Debug.Assert(INIT_SEGMENT_SIZE <= MAX_SEGMENT_SIZE, "Initial segment size should be <= maximum.");
			Debug.Assert(MAX_SEGMENT_SIZE < Int32.MaxValue / 2, "Max segment size * 2 must be < Int32.MaxValue, or else overflow could occur.");

			// Initialize the queue
			_head = _tail = new Segment(INIT_SEGMENT_SIZE);
		}

		/// <summary>Enqueues an item into the queue.</summary>
		/// <param name="item">The item to enqueue.</param>
		public void Enqueue(T item)
		{
			Segment segment = _tail;
			T[] array = segment._array;
			int last = segment._state._last; // local copy to avoid multiple volatile reads

			// Fast path: there's obviously room in the current segment
			int tail2 = (last + 1) & (array.Length - 1);
			if (tail2 != segment._state._firstCopy)
			{
				array[last] = item;
				segment._state._last = tail2;
			}
			// Slow path: there may not be room in the current segment.
			else EnqueueSlow(item, ref segment);
		}

		/// <summary>Enqueues an item into the queue.</summary>
		/// <param name="item">The item to enqueue.</param>
		/// <param name="segment">The segment in which to first attempt to store the item.</param>
		private void EnqueueSlow(T item, ref Segment segment)
		{
			Contract.Requires(segment != null, "Expected a non-null segment.");

			if (segment._state._firstCopy != segment._state._first)
			{
				segment._state._firstCopy = segment._state._first;
				Enqueue(item); // will only recur once for this enqueue operation
				return;
			}

			int newSegmentSize = _tail._array.Length << 1; // double size
			Debug.Assert(newSegmentSize > 0, "The max size should always be small enough that we don't overflow.");
			if (newSegmentSize > MAX_SEGMENT_SIZE) newSegmentSize = MAX_SEGMENT_SIZE;

			var newSegment = new Segment(newSegmentSize);
			newSegment._array[0] = item;
			newSegment._state._last = 1;
			newSegment._state._lastCopy = 1;

			try { }
			finally
			{
				// Finally block to protect against corruption due to a thread abort 
				// between setting _next and setting _tail.
				Volatile.Write(ref _tail._next, newSegment); // ensure segment not published until item is fully stored
				_tail = newSegment;
			}
		}

		/// <summary>Attempts to dequeue an item from the queue.</summary>
		/// <param name="result">The dequeued item.</param>
		/// <returns>true if an item could be dequeued; otherwise, false.</returns>
		public bool TryDequeue(out T result)
		{
			Segment segment = _head;
			T[] array = segment._array;
			int first = segment._state._first; // local copy to avoid multiple volatile reads

			// Fast path: there's obviously data available in the current segment
			if (first != segment._state._lastCopy)
			{
				result = array[first];
				array[first] = default(T); // Clear the slot to release the element
				segment._state._first = (first + 1) & (array.Length - 1);
				return true;
			}
			// Slow path: there may not be data available in the current segment
			else return TryDequeueSlow(ref segment, ref array, out result);
		}

		/// <summary>Attempts to dequeue an item from the queue.</summary>
		/// <param name="array">The array from which the item was dequeued.</param>
		/// <param name="segment">The segment from which the item was dequeued.</param>
		/// <param name="result">The dequeued item.</param>
		/// <returns>true if an item could be dequeued; otherwise, false.</returns>
		private bool TryDequeueSlow(ref Segment segment, ref T[] array, out T result)
		{
			Contract.Requires(segment != null, "Expected a non-null segment.");
			Contract.Requires(array != null, "Expected a non-null item array.");

			if (segment._state._last != segment._state._lastCopy)
			{
				segment._state._lastCopy = segment._state._last;
				return TryDequeue(out result); // will only recur once for this dequeue operation
			}

			if (segment._next != null && segment._state._first == segment._state._last)
			{
				segment = segment._next;
				array = segment._array;
				_head = segment;
			}

			int first = segment._state._first; // local copy to avoid extraneous volatile reads

			if (first == segment._state._last)
			{
				result = default(T);
				return false;
			}

			result = array[first];
			array[first] = default(T); // Clear the slot to release the element
			segment._state._first = (first + 1) & (segment._array.Length - 1);
			segment._state._lastCopy = segment._state._last; // Refresh _lastCopy to ensure that _first has not passed _lastCopy

			return true;
		}

		/// <summary>Attempts to peek at an item in the queue.</summary>
		/// <param name="result">The peeked item.</param>
		/// <returns>true if an item could be peeked; otherwise, false.</returns>
		public bool TryPeek(out T result)
		{
			Segment segment = _head;
			T[] array = segment._array;
			int first = segment._state._first; // local copy to avoid multiple volatile reads

			// Fast path: there's obviously data available in the current segment
			if (first != segment._state._lastCopy)
			{
				result = array[first];
				return true;
			}
			// Slow path: there may not be data available in the current segment
			else return TryPeekSlow(ref segment, ref array, out result);
		}

		/// <summary>Attempts to peek at an item in the queue.</summary>
		/// <param name="array">The array from which the item is peeked.</param>
		/// <param name="segment">The segment from which the item is peeked.</param>
		/// <param name="result">The peeked item.</param>
		/// <returns>true if an item could be peeked; otherwise, false.</returns>
		private bool TryPeekSlow(ref Segment segment, ref T[] array, out T result)
		{
			Contract.Requires(segment != null, "Expected a non-null segment.");
			Contract.Requires(array != null, "Expected a non-null item array.");

			if (segment._state._last != segment._state._lastCopy)
			{
				segment._state._lastCopy = segment._state._last;
				return TryPeek(out result); // will only recur once for this peek operation
			}

			if (segment._next != null && segment._state._first == segment._state._last)
			{
				segment = segment._next;
				array = segment._array;
				_head = segment;
			}

			int first = segment._state._first; // local copy to avoid extraneous volatile reads

			if (first == segment._state._last)
			{
				result = default(T);
				return false;
			}

			result = array[first];
			return true;
		}

		/// <summary>Attempts to dequeue an item from the queue.</summary>
		/// <param name="predicate">The predicate that must return true for the item to be dequeued.  If null, all items implicitly return true.</param>
		/// <param name="result">The dequeued item.</param>
		/// <returns>true if an item could be dequeued; otherwise, false.</returns>
		public bool TryDequeueIf(Predicate<T> predicate, out T result)
		{
			Segment segment = _head;
			T[] array = segment._array;
			int first = segment._state._first; // local copy to avoid multiple volatile reads

			// Fast path: there's obviously data available in the current segment
			if (first != segment._state._lastCopy)
			{
				result = array[first];
				if (predicate == null || predicate(result))
				{
					array[first] = default(T); // Clear the slot to release the element
					segment._state._first = (first + 1) & (array.Length - 1);
					return true;
				}
				else
				{
					result = default(T);
					return false;
				}
			}
			// Slow path: there may not be data available in the current segment
			else return TryDequeueIfSlow(predicate, ref segment, ref array, out result);
		}

		/// <summary>Attempts to dequeue an item from the queue.</summary>
		/// <param name="predicate">The predicate that must return true for the item to be dequeued.  If null, all items implicitly return true.</param>
		/// <param name="array">The array from which the item was dequeued.</param>
		/// <param name="segment">The segment from which the item was dequeued.</param>
		/// <param name="result">The dequeued item.</param>
		/// <returns>true if an item could be dequeued; otherwise, false.</returns>
		private bool TryDequeueIfSlow(Predicate<T> predicate, ref Segment segment, ref T[] array, out T result)
		{
			Contract.Requires(segment != null, "Expected a non-null segment.");
			Contract.Requires(array != null, "Expected a non-null item array.");

			if (segment._state._last != segment._state._lastCopy)
			{
				segment._state._lastCopy = segment._state._last;
				return TryDequeueIf(predicate, out result); // will only recur once for this dequeue operation
			}

			if (segment._next != null && segment._state._first == segment._state._last)
			{
				segment = segment._next;
				array = segment._array;
				_head = segment;
			}

			int first = segment._state._first; // local copy to avoid extraneous volatile reads

			if (first == segment._state._last)
			{
				result = default(T);
				return false;
			}

			result = array[first];
			if (predicate == null || predicate(result))
			{
				array[first] = default(T); // Clear the slot to release the element
				segment._state._first = (first + 1) & (segment._array.Length - 1);
				segment._state._lastCopy = segment._state._last; // Refresh _lastCopy to ensure that _first has not passed _lastCopy
				return true;
			}
			else
			{
				result = default(T);
				return false;
			}
		}

		public void Clear()
		{
			T ignored;
			while (TryDequeue(out ignored)) ;
		}

		/// <summary>Gets whether the collection is currently empty.</summary>
		/// <remarks>WARNING: This should not be used concurrently without further vetting.</remarks>
		public bool IsEmpty
		{
			// This implementation is optimized for calls from the consumer.
			get
			{
				Segment head = _head;
				if (head._state._first != head._state._lastCopy) return false; // _first is volatile, so the read of _lastCopy cannot get reordered
				if (head._state._first != head._state._last) return false;
				return head._next == null;
			}
		}

		/// <summary>Gets an enumerable for the collection.</summary>
		/// <remarks>WARNING: This should only be used for debugging purposes.  It is not safe to be used concurrently.</remarks>
		public IEnumerator<T> GetEnumerator()
		{
			for (Segment segment = _head; segment != null; segment = segment._next)
			{
				for (int pt = segment._state._first;
						pt != segment._state._last;
						pt = (pt + 1) & (segment._array.Length - 1))
				{
					yield return segment._array[pt];
				}
			}
		}
		/// <summary>Gets an enumerable for the collection.</summary>
		/// <remarks>WARNING: This should only be used for debugging purposes.  It is not safe to be used concurrently.</remarks>
		IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }

		/// <summary>Gets the number of items in the collection.</summary>
		/// <remarks>WARNING: This should only be used for debugging purposes.  It is not meant to be used concurrently.</remarks>
		public int Count
		{
			get
			{
				int count = 0;
				for (Segment segment = _head; segment != null; segment = segment._next)
				{
					int arraySize = segment._array.Length;
					int first, last;
					while (true) // Count is not meant to be used concurrently, but this helps to avoid issues if it is
					{
						first = segment._state._first;
						last = segment._state._last;
						if (first == segment._state._first) break;
					}
					count += (last - first) & (arraySize - 1);
				}
				return count;
			}
		}

		/// <summary>A thread-safe way to get the number of items in the collection. May synchronize access by locking the provided synchronization object.</summary>
		/// <remarks>The Count is not thread safe, so we need to acquire the lock.</remarks>
		int IProducerConsumerQueue<T>.GetCountSafe(object syncObj)
		{
			Debug.Assert(syncObj != null, "The syncObj parameter is null.");
			lock (syncObj)
			{
				return Count;
			}
		}

		/// <summary>A segment in the queue containing one or more items.</summary>
		[StructLayout(LayoutKind.Sequential)]
		private sealed class Segment
		{
			/// <summary>The next segment in the linked list of segments.</summary>
			internal Segment _next;
			/// <summary>The data stored in this segment.</summary>
			internal readonly T[] _array;
			/// <summary>Details about the segment.</summary>
			internal SegmentState _state; // separated out to enable StructLayout attribute to take effect

			/// <summary>Initializes the segment.</summary>
			/// <param name="size">The size to use for this segment.</param>
			internal Segment(int size)
			{
				Contract.Requires((size & (size - 1)) == 0, "Size must be a power of 2");
				_array = new T[size];
			}
		}

		/// <summary>Stores information about a segment.</summary>
		[StructLayout(LayoutKind.Sequential)] // enforce layout so that padding reduces false sharing
		private struct SegmentState
		{
			/// <summary>Padding to reduce false sharing between the segment's array and _first.</summary>
			internal PaddingFor32 _pad0;

			/// <summary>The index of the current head in the segment.</summary>
			internal volatile int _first;
			/// <summary>A copy of the current tail index.</summary>
			internal int _lastCopy; // not volatile as read and written by the producer, except for IsEmpty, and there _lastCopy is only read after reading the volatile _first

			/// <summary>Padding to reduce false sharing between the first and last.</summary>
			internal PaddingFor32 _pad1;

			/// <summary>A copy of the current head index.</summary>
			internal int _firstCopy; // not volatile as only read and written by the consumer thread
			/// <summary>The index of the current tail in the segment.</summary>
			internal volatile int _last;

			/// <summary>Padding to reduce false sharing with the last and what's after the segment.</summary>
			internal PaddingFor32 _pad2;
		}

		/// <summary>Debugger type proxy for a SingleProducerSingleConsumerQueue of T.</summary>
		private sealed class SingleProducerSingleConsumerQueue_DebugView
		{
			/// <summary>The queue being visualized.</summary>
			private readonly SingleProducerSingleConsumerQueue<T> _queue;

			/// <summary>Initializes the debug view.</summary>
			/// <param name="queue">The queue being debugged.</param>
			public SingleProducerSingleConsumerQueue_DebugView(SingleProducerSingleConsumerQueue<T> queue)
			{
				Contract.Requires(queue != null, "Expected a non-null queue.");
				_queue = queue;
			}

			/// <summary>Gets the contents of the list.</summary>
			[DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
			public T[] Items
			{
				get
				{
					List<T> list = new List<T>();
					foreach (T item in _queue)
						list.Add(item);
					return list.ToArray();
				}
			}
		}
	}


	/// <summary>A placeholder class for common padding constants and eventually routines.</summary>
	static class PaddingHelpers
	{
		/// <summary>A size greater than or equal to the size of the most common CPU cache lines.</summary>
		internal const int CACHE_LINE_SIZE = 128;
	}

	/// <summary>Padding structure used to minimize false sharing in SingleProducerSingleConsumerQueue{T}.</summary>
	[StructLayout(LayoutKind.Explicit, Size = PaddingHelpers.CACHE_LINE_SIZE - sizeof(Int32))] // Based on common case of 64-byte cache lines
	struct PaddingFor32
	{
	}
}